Bleach (sodium hypochlorite) is a commodity chemical that is used in numerous applications. The basic chemistry for manufacturing bleach is a matter of common knowledge in the fields of chemistry and chemical engineering. Chlorine in gas and/or liquid phase is allowed to react with a solution of sodium hydroxide (caustic) to yield aqueous sodium hypochlorite. While that basic chemistry may be considered rather elementary, and essentially common to all processes for the commercial manufacture of bleach, specific processes that have been described in patent literature differ in significant ways.
Each of the various known processes for the commercial manufacture of bleach may be characterized as either a batch (discontinuous) production process or a continuous production process. Each type of process may have its own particular advantages.
A continuous process that is properly controlled is more likely to be performed with higher production efficiency than a corresponding batch process, and hence is likely to be more economical than a batch process. However, the specific manner in which a continuous process is performed plays a significant role in the nature and quality of the resulting bleach product.
U.S. Pat. Nos. 4,428,918 and 4,780,303 each describes a respective continuous process for manufacture of concentrated (i.e. high-strength) sodium hypochlorite solutions. Sodium chloride (salt) is however also a product of the basic reaction, and its removal from the aqueous sodium hypochlorite product can improve both the continuous process and the resulting product. Neither of those processes removes all of the salt from the resulting product.
According to U.S. Pat. No. 4,428,918, a fraction of the suspension from a central reaction chamber overflows the top of an inner cylindrical wall surrounding the reaction chamber inside a tank to fall into an annular space between the inner wall and the sidewall of the tank. Near the top of the annular space some of that fraction is diverted from the tank to filtration apparatus while the balance passes downwardly through the annular space to the bottom of the tank where it is re-cycled into the reaction chamber. The patent further says that in the filtration apparatus, a filter cake is separated, containing, by weight, 80.1% of sodium chloride and 19.9% of sodium hypochlorite and 1775 kg per hour of an aqueous solution of sodium hypochlorite is recovered, containing, per kg, 257 g of sodium hypochlorite and 94 g of dissolved sodium chloride. The ultimate disposition of the salt cake is not discussed.
According to U.S. Pat. No. 4,780,303, sodium hypochlorite solution leaves the top of a crystallizer tank at the rate of about 14 kg/hr and contains about 25% NaOCl, 9.5% of sodium chloride, as well as a slight excess of sodium hydroxide. The reader is led to understand that it is that solution that the patent considers as the high strength bleach product. The patent further mentions that precipitated salt is removed, continuously or intermittently, from the crystallizer tank through conduit at the bottom of the tank. The mean crystal size of the sodium chloride removed is said to be in the neighborhood of 400 microns, a size that facilitates rapid filtration and reduces the retention of mother liquor in the salt so that there is very small sodium hypochlorite retention in the sodium chloride after subsequent filtration.
Both patents recognize that certain batch processes can produce aqueous high-strength bleach from which significant amounts of salt have been removed.
It is believed that a continuous process that is capable of consistently producing aqueous high-strength bleach with low concentrations of both sodium chloride and sodium chlorate with residual slight excess caustic would be beneficial to industry. A product that has even greater strength, and lower salt and chlorate concentrations, than those mentioned in U.S. Pat. Nos. 4,428,918 and 4,780,303 would be especially beneficial. The benefits reside both in the utility of the product and relevant economic factors.
The processes that are described in the inventors' priority provisional and non-provisional patent applications employ a crystallizer stage in which the reactants create a salt slurry in a bottom zone of a crystallizer tank. As fresh solutions of lower-strength bleach and caustic continuously enter into solution in the tank, the salt slurry at the bottom is being continuously pumped out.
A first portion of the withdrawn slurry forms a recycle solution that is cooled during passage through a heat exchanger before being fed back into the crystallizer tank. The fresh caustic is entrained with the recycle solution ahead of the heat exchanger. Fresh lower-strength bleach is entrained with the entrained caustic and recycle solution after the heat exchanger.
The crystallizer stage tank shown in the two earliest priority patent applications comprises a skirt baffle that is inside the cylindrical sidewall of the tank and forms a cylindrical wall to create an annular calming zone between the skirt baffle and the tank sidewall. The annular calming zone is essentially free of turbulence, especially toward the top where an upper zone of essentially crystal-free mother liquor is created. The skirt baffle surrounds a central inner zone into which the fresh bleach and caustic and the recycle solution are introduced. The annular calming zone and the central inner zone are both above and open to the bottom zone.
Continually overflowing mother liquor at an appropriate rate from the top of the calming zone supersaturates the solution resulting in salt continuously precipitating out of solution with the salt crystals continuously replenishing the slurry in the bottom zone.
The third priority patent application Ser. No. 11/648,411 relates to a simplification of the equipment that eliminates the skirt baffle inside the crystallizer stage tank. The continuous process is conducted in a crystallizer tank without continuously drawing off mother liquor at the top of a calming zone. That allows the tank to not have a skirt baffle that otherwise would divide the portion of the tank above the bottom zone into a central inner zone surrounded by an outer calming zone.
The higher-strength, lower-salt bleach products that can be manufactured are those which comprise an aqueous solution of greater than 25% by weight sodium hypochlorite and a ratio, on a % weight basis, of NaCl (salt) to NaOCl (sodium hypochlorite), less than substantially 0.38, with a slight excess of sodium hydroxide (caustic). After removal of solids, a bleach having approximately 30% to approximately 35% by weight sodium hypochlorite and a NaCl/NaOCl ratio of about 0.21 to about 0.25 at 30% strength and about 0.10 to about 0.15% at 35% strength, with slight excess caustic, is one example of such a higher-strength, lower-salt bleach product.
A solution of the fresh caustic, the fresh lower-strength bleach that is essentially free of salt crystals, and salt slurry withdrawn from the bottom zone of the tank to form the recycle solution are continuously introduced into the crystallizer tank. The solution in the tank is chlorinated by introducing chlorine in liquid and/or gas phase, wet or dry, with or without inerts. The percentage of excess caustic in solution is controlled in any suitable manner using an appropriate measurement, such as oxidation-reduction potential measurement, by commercially available equipment.
A heat exchanger associated with the crystallizer tank removes heats of solution and reaction from slurry that is withdrawn from the tank to form the recycle solution. By using a high recycle rate through the heat exchanger, the temperature drop between the recycle outlet from the tank and the recycle return to the tank can be kept small, a benefit that aids crystal formation while avoiding fouling of the heat exchanger. A temperature drop within a range from about 1° F. to about 4° F. would be typical, with a range from about 1° F. to about 2° F. being most preferable.
In order to control the temperature drop to within such a range, the heat exchanger is one that has sufficient heat transfer surface area in relation to the flow rates of the respective liquids passing through it and that presents low restriction to the flows. By suitable control of chemical processes in the crystallizer stage, the temperature of those processes can be kept within a range that allows sometimes allows cooling tower water to be used as the cooling liquid in certain types of heat exchangers, avoiding the need to use more expensive refrigerated water in those instances.
The temperature difference between the recycle solution and the cooling liquid passing through the heat exchanger is also controlled. A target range of temperature differences that avoid fouling of the heat exchanger depends on the design of the particular heat exchanger. For a plate and frame type heat exchanger, the temperature difference may have a range of from about 2° F. to about 3° F. The temperature difference may have a larger range, 5° F.-15° F. for example, for other heat exchangers, such as a shell and tube type.
The fresh caustic is preferably added to the recycle solution ahead of the heat exchanger. The caustic is itself preferably cooled by passage through its own heat exchanger before being added to the recycle solution. The fresh lower-strength bleach is added to entrain with the entrained caustic and recycle solution after the latter have been cooled.
With the process continuously running, the continuous introduction of chlorine and the mixture of recycle solution, caustic, and lower-strength bleach sustains a continuous reaction in the tank to produce higher-strength bleach. The mixture of recycle solution, fresh caustic, and fresh lower-strength bleach, is introduced into solution already in the tank at a level above a bottom zone from which the recycle solution is being withdrawn. The chlorine is also introduced above the bottom zone, and above the level at which the liquid mixture is being introduced.
The level to which solution fills in the tank is controlled, or regulated, in any suitable manner, by process control apparatus. No solution overflows the tank or is withdrawn from the tank at a level above the bottom zone. Solution that is withdrawn from the bottom zone as a slurry becomes either recycle solution returned to the tank or is subsequently processed to yield the final higher-strength, lower-salt bleach product. In the absence of a distinct calming zone in the crystallizer tank as described in the priority patent applications, the solution in the tank is generally homogeneous, especially in the bottom zone.
Process control conditions can create sufficiently large crystal sizes for effective removal of crystals by mechanical processes with the resulting distribution of salt crystal sizes in the slurry rendering them well-suited for ultimate recovery as essentially dry solids.